Every electrical or electronic device produces electrical fields and therefore emits radiation as a result of electrical activity. In most cases such a radiation is not desired since it may cause interferences and it may affect, i.e. disturb or even damage electrical devices. The radiation may either affect the device itself which emits the radiation or it may affect other devices in the vicinity of the emitting device. Additionally, also natural phenomena such as for example lightning of a thunderstorm may be a source of unwanted electromagnetic radiation which may affect an electrical or electronic device. In order to reduce or even eliminate the effect of such radiation electrical devices or often shielded. This is for example done by encasing a device with an electrically conductive material such as a metal connected to ground. With such a Faraday cage it is possible to prevent electromagnetic radiation that has been produced externally to enter the device or electromagnetic radiation that has been produced internally to leave the device. Often, such electrical devices have a casing which is made of a metal and is therefore not only used as the housing but at the same time also as the shielding of the device.
However, in many cases such devices need power that has to be supplied to the device or the device produces electrical power that has to be supplied to other devices. In another example, other electrical signals such as for example signals for data transmission or the like have to be delivered to or lead away from the device. Accordingly, the shielding has to have an opening through which such power or other signals can be fed, most often by means of corresponding cables. Through such a hole, electromagnetic radiation may however undesirably enter or leave the device.
In order to reduce the passage of electromagnetic radiation through such a hole, the hole may be provided with EMI shielding devices, so called EMI gaskets. An EMI gasket typically is made of an electrically conductive material and is mounted all around the edges of the opening to completely surround the opening. To do so, the gaskets are formed as a clamp or they do have a clamp part with which the gaskets are clamped onto an edge of a casing wall. Such EMI gaskets are usually marketed in the form of long strips that are built from a plurality of elements that are arranged in a row and where two neighboring elements are interconnected. Such gaskets, in particular when made of metal, are usually manufactured by cutting the interconnected elements from a flat, thin piece of metal and then applying the necessary bends, arcs, curves etc. to provide the desired shape. The cutting is done by punching, die-cutting or otherwise carving the required basic shape of the gasket.
For mounting the gaskets, the strips are cut into pieces to fit the edge where the gasket is to be mounted and then clamping the cut gasket onto the respective edge.
The shape of the gasket is chosen such as to minimize the passage of radiation. There is however not a particular shape of gasket that works well in different applications. Rather the shape of a gasket has to be adapted to and optimized in view of a particular application.
Such an EMI gasket may for example be made of a metal but EMI gaskets made of electrically conductive foam or another conductive material are also known. There are also different methods to attach the gasket to the casing. Whereas metallic gaskets are usually clamped onto the casing wall, foam gaskets are generally glued to the casing. Although foam gaskets usually do not perform as good as metal gaskets, metal gaskets may not be used in certain applications such as for example where loss prevention gaskets have to be used. Although foam gaskets are cheaper than metal gaskets, glued foam gaskets are weaker than metal gaskets and foam gaskets can easily be damaged and replacement is difficult because they are glued. Foam gaskets are aging and they do not resist high temperatures.
Accordingly, metal gaskets are generally preferred. But one major disadvantage of existing metal gaskets is that the gaskets mounted on a housing or a casing may get out of place or even slip off the base unintentionally because the clamping is not very strong. Moreover, the clamping must not be too strong because it should also be possible to voluntarily remove the gaskets if required.
But even if it would be acceptable to have a much stronger clamping of the gaskets, the gaskets would have to be made of a much stronger or thicker material which is not very useful in most applications.